1. Field of the Invention
This invention relates to film processors having a film density detector arrangement including a light source and a photodetector the output of which is used to generate a reference background level signal against which the transmissivity of processed film is compared and, in particular, to an automatic monitoring arrangement which samples the instantaneous output of the film density detector arrangement as a function of film transport speed and adjusts the reference background level signal to accommodate fluctuations in the output of the film density detector.
2. Description of the Prior Art
Film processors representative of the known art typically include developing, fixing, washing and drying sections through which a film to be processed is transported on an array of transport rollers. The transport rollers are typically driven by a drive motor at predetermined development speed selected so that the film remains within the processor (in particular, the developing section thereof) for a predetermined ideal development time. It is within the developing and fixing sections that the chemical activity generated by the chemical baths disposed within these sections develops and fixes the image on the exposed film. Thereafter, the film is washed and dried prior to its exit from the processor.
During the developing and fixing of the images on the film the constituent chemicals in the developing and processing baths may become depleted. It is, therefore, necessary to periodically replenish the chemicals within these baths in order to maintain their efficacy.
It has been the practice in the art to provide automated developing and fixing chemical replenishment control systems. Such control systems usually include a timing network adapted to control the period during which pumps or other suitable apparatus introduce replenishing supplies of developing and/or fixing chemicals to the baths in order to return the chemical levels within the developing and fixing sections to predetermined concentrations.
The timing network is usually responsive to an initiating signal output from a film density detector. The density detector acts to inspect each processed film at a point just past the drying section and just before the processor outlet in order to obtain an indication of the amount of developing and fixing chemicals used to develop and fix the images on that particular film. The density detector usually operates by generating signals representative of the film's transmissivity which, in turn, is representative of the optical density of the exposed and developed photosensitive layers on the film. The optical film density provides an indication as to the amount of chemical used to develop and fix the image on that film. The information as to the amount of chemical developing and chemical fixing solution utilized is accumulated and, when the accumulated signal exceeds a predetermined threshold, the initiating signal to the timing network is generated.
The density detector typically includes a source of light disposed on one side of the path of the processed film and a photodetector disposed on the opposite side of the film in a position opposite to the light source. The photodetector generates an output signal that is functionally related to the intensity of the light transmitted through the processed film and incident upon the photodetectors. The change in output signal of the photodetectors from a background reference level derived when no film is interposed between the source and the photodetector provides an indication as to the amount of developing and fixing chemicals which are utilized during the processing of the particular film.
The reference background level of the photodetector is established when no film is interposed between the source and the photodetector. Accordingly, the reference background level is related to the intensity of the light source and the sensitivity of the photodetector. Thus, the response when no film is within the device is subject to fluctuations caused by various factors. Over time, factors as line transients, dust, aging, among others, may have the effect of varying the intensity of the light source.
Additionally, some of the more recent film processors are provided with an arrangement whereby the drying section is deenergized and permitted to cool during those periods when no film is being processed within the processor. However, when a film is inserted into the processor it is necessary to again heat up the drying section to the appropriate drying temperature. The temperature excursions imposed upon the density detector arrangement due to its physical proximity to the drying section also have an effect upon the measurement of the density detector output.
The art has recognized the need to periodically monitor the reference background level in order to maintain a calibrated system. The monitoring attempts in the prior art have usually included periodic inspections by maintenance personnel in order to ascertain if deviations from a predetermined reference background level have occurred. The typical response in such an instance is to adjust the gain in the photodetector circuit or, perhaps the intensity of the source, so that the reference background level is restored to the predetermined value. The frequency of such periodic monitoring tasks is unrelated to the operation of the processor and is usually dependent upon the work schedule of the monitoring technician. It is ordinarily not responsive to the relatively short range thermal excursions of the processor.
It is believed to be advantageous to provide an automatic monitoring system adapted to periodically sample and respond to the instantaneous photodetector signal in order to accommodate fluctuations in the density detector arrangement. It is also believed to be advantageous to provide a monitoring system which automatically responds to a measured deviation in the photodetector signal level by adjusting the response to the value of that level, rather than modifying the gain of the photodetector or the intensity of the light source. It is believed to be of further advantage to sample and to respond to the instantaneous photodetector output in accordance with the transport speed of the processor, rather than in accordance with a sampling schedule unrelated to the operation of the processor. It is believed to be of further advantage to respond only to established trends in reference background level and not to minor transients. It would be of still further advantage to implement the automatic monitoring arrangement through the use of a digital computer operating in accordance with its program and most preferably, through the use of a firmware-based microcomputer arrangement.